The present invention relates to a gas laser comprising a container intended to receive a gas or a gas mixture as laser-active medium, the container having an axis along which the gas laser emits its laser radiation, and comprising further means for generating a plasma in the container and means for generating a magnetic field in the container. A gas laser of this kind is known from xe2x80x9cLaserxe2x80x9d by F. K. Kneubxc3xchl, M. W. Sigirst, published by B. G. Teubner Stuttgart, 1988, pp. 232 to 271.
More specifically, the invention concerns rare-gas lasers, for example helium-neon lasers, ion lasers, especially argon ion lasers, and molecular lasers, especially CO2 lasers.
Excitation of the active medium in a gas laser usually is initiated by an electric discharge. Free electrons and ions are produced in an electric gas discharge. These charge carriers gain kinetic energy due to the acceleration in the electric field of the gas discharge. The kinetic energy of the electrons so gained can be transferred, by inelastic collision, to other gas particles and can excite the latter to higher levels. The movement of the ions is, generally, of no importance as only the free electrons contribute to the excitation of the gas atoms, gas ions or molecules. A stimulating emission of radiation can then take place from the higher levels.
A helium neon laser is a neutral atom gas laser in which a direct-current or high-frequency discharge is excited in order to obtain a laser-active plasma. The efficiency of the laser, defined as quotient of optic performance and electric power input, is, typically, as low as 0.1%. This is due, among other things, to the rather inefficient excitation mechanism (Kneubxc3xchl/Sigrist, loc. cit., p. 240).
A typical argon ion laser comprises, in a vessel configured as a tube, a cascade-like electrode arrangement in which a high-intensity arc discharge is excited between the electrodes in the gas filling, which typically has a gas pressure of 0.01-1 mbar, in order to obtain a laser-active plasma with a degree of ionisation in an order of magnitude of 10xe2x88x924 to 10xe2x88x922. The efficiency of an argon ion laser is likewise very low, being less than 0.1%. The beam quality, for example the beam jitter and its maximum intensity, depend on the age of the tube. Due to interaction between the plasma and the tube, the life of the tube is heavily restricted, being typically only 2000 hours for a laser power of 20 W. At the end of that time, the tube, which costs some ten thousand D-marks, must be replaced. In addition, the tube requires intensive cooling, the cooling power being typically in the range of up to 40 kW. It has been known in connection with an argon ion laser to apply a longitudinal magnetic field, i.e. a magnetic field that extends in parallel to the longitudinal axis of the laser tube, in order to concentrate the discharge on the axis and to reduce the damaging effects the plasma has on the tube wall (Kneubxc3xchl/Sigrist, loc. cit. p. 246). However, this measure is only little effective and has not succeeded in increasing the efficiency of the laser to over 0.1%.
CO2 lasers are excited by direct-current discharges or electromagnetic fields in the radio-frequency range, depending on the particular design. Their efficiency is, typically, between 10% and 15%, depending on the particular design and operating mode.
The present invention has for its object to improve the efficiency of gas lasers and extend the life of the container that encloses the laser-active medium.
This object is achieved by a gas laser having the features defined in claim 1. Advantageous further improvements of the invention are the subject-matter of the sub-claims.
Gas lasers comprise a container intended to receive a gas or a gas mixture as laser-active medium, the container having an axis along which the gas laser emits its laser radiation. For exciting the laser-active medium, a source, associated to the container, of an electromagnetic alternating field is used to produce a plasma in which the electromagnetic alternating field is injected into the laser-active medium in the container. For producing a magnetic field in the container, a group of at least two pairs of magnet poles is provided, with the poles extending at a distance from, and along the axis of the container and being arranged around the axis with alternating polarity so that the magnetic field emanating from them exhibits an intensity sink in the region of the axis. As a result, a plasma column is produced which represents the essential portion of the laser-active volume, the geometric dimensions of which are determined by the confinement of the plasma in a static or slowly variable magnetic multipole field. Using energy irradiated and/or injected from the electromagnetic alternating field, electric fields with azimuthal or axially parallel components are produced that cause the plasma to become denser in the magnetic multipole field.
This provides the following essential advantages:
Due to the interaction between the charge carriers present in the plasma and the magnetic field and the electromagnetic alternating field, the charge carriers, especially the electrons are driven away from the container wall and toward the container axis, where the magnetic field exhibits an intensity sink and, preferably, disappears. This greatly reduces interaction between the plasma and the container wall.
The reduced interaction between the plasma and the container wall has the result to extend the life of the container.
As a result of the reduced interaction between the plasma and the container wall, the degree of heating-up of the latter, and thus the cooling power required, are reduced.
The reduced interaction between the plasma and the container wall reduces the level of absorption of gas components by the container wall and, as a result thereof, maintains the optimum composition and the optimum pressure of the gas over a longer period of time.
Due to the reduced interaction between the electrons of the plasma and the container wall, more electrons, and electrons of higher energy are available for gas-exciting collisions, whereby the efficiency of the laser is increased.
Because the electrons of the plasma are driven into the intensity sink of the magnetic field, the electron density and the collision probability of the electrons increases in that region so that the light yield and, with in, the efficiency of the laser, increase. First test have shown that compared with the prior art the efficiency of, for example, an argon ion laser can be increased by a factor of ten to twenty.
As a result of the concentration of the excitation-triggering electrons in the near environment of the axis of the container, the zone emitting the laser radiation becomes narrower so that the laser beam becomes thinner and more intensive.
As a result of the reduced interaction between the plasma and the container wall and the increase of the electron density in the near environment of the container axis, a rise in electron temperature (kinetic energy of the electrons) occurs in this region, which temperature may be further increased by an increase in amplitude of the electromagnetic alternating field. The increase in amplitude of the electromagnetic alternating field in turn is facilitated by the reduced interaction between the plasma and the container wall.
The invention renders possible a higher electron temperature, which in turn permits the excitation of higher energy levels and, thus, laser radiation with shorter wave lengths, down to the roentgen range.
The high electron temperature, that can be reached with a relatively low HF power, leads to a considerably lower build-up threshold of the laser, from an energetic point of view.
The invention permits plasmas to be generated in the laser container, in which the electron temperature is much higher than the ion temperature, which is desirable as such.
The reduced interaction between the plasma and the container wall permits the use of containers made from materials that are less resistant, less temperature-resistant, less toxic and cheaper than beryllium oxide, the material frequently used for argon ion lasers.
The effectiveness of the magnetic field, which in combination with the electromagnetic alternating field concentrates the plasma and, especially, its electrons in the intensity sink of the magnetic field, rises with its gradient transverse to the container axis. It is, therefore, favourable if the magnetic field disappears in the region of the container axis, while being as high as possible at the edge of the container, the intensity of the magnetic field being solely limited by technical constraints in connection with the generation of the magnetic field and by financial constraints. Magnetic fields with an intensity of more than 10 Tesla will not be used for cost reasons; magnetic fields with an intensity of between 0.1 and 2 Tesla at the pole shoe surface, especially between 0.1 and 1 Tesla, represent a good compromise between the strength of the magnetic field and the cost of its generation.
While in principle the magnetic poles may be located within the laser container, they are, preferably, located outside the container. In principle, the magnet may be a permanent magnet. Because of the high field strengths achievable, electromagnets are, however, preferred. These may be excited by direct current or alternating current, especially by alternating current which is available with 50 Hz as a standard in Europe, and with 60 Hz as a standard in the USA so that a low-cost magnet structure can be realised. In cases where the electromagnets are excited by alternating current, rather than by direct current, the frequency of the exciting alternating current should, however, be small compared with the frequency of the electromagnetic alternating field used to generate the plasma and to excite the laser-active medium, because the desired focusing effect of the magnetic field is optimally achieved if it is slowly variable only, or quasi-static, compared with the frequency of the electromagnetic alternating field used for generating the plasma.
Conveniently, the electrons of the plasma are concentrated, by their interaction with the magnetic field and with the electromagnetic alternating field, in a substantially straight-line region, with the laser radiation being emitted in the latter""s lengthwise direction. It is, therefore, preferred that the axis, in the near environment of which the electrons are concentrated, is both an axis of symmetry of the arrangement of the pairs of magnet poles and an axis of symmetry of the container.
In the simplest case, the container is a tube, especially one of circular cross-section. However, tubes with rectangular or square cross-section or even with a cross-section corresponding to the cross-section of the radiant plasma column that results from the interaction of the plasma with the magnetic field and the electromagnetic alternating field, are also possible (see for example FIG. 2).
According to the preferred embodiment of the invention, a magnetic four-pole (quadrupole) field is generated which exhibits an intensity sink on the axis of the laser container. In this case, two pairs of magnet poles are arranged, preferably symmetrically, around the container axis. However, it is also possible to work with a six-pole field or with an eight-pole field. Using more than eight magnet poles arranged around the axis causes additional costs, but brings no relevant advantage so that it is preferred to arrange maximally eight poles (four pairs of magnet poles) around the axis.
The magnets may have pole shoes extending continuously over the entire length of the space in the laser container provided for excitation of the gas. In case of greater lengths, it may however be more economic and easier in technical terms to arrange several groups of magnet pole pairs one behind the other along the container axis, in which case such groups should, conveniently, be identical one to the other.
The electromagnetic alternating field, which has the function to generate a plasma whose electrons are capable of absorbing sufficient energy in the electromagnetic alternating field for exciting the desired energy level in the laserable gas, can be generated in different ways. One of such ways consists in using, as a source of the electromagnetic alternating field, a coil that surrounds the container and that is part of an electric oscillating circuit which latter is fed by a frequency generator. Preferably, the coil surrounds the container only over part of its length so that the plasma, being stimulated to radiate, is kept free not only of the circumferential wall of the laser container, but also of its two end walls, in order to minimise also the stresses acting on the end walls of the container and to minimise the losses of charge carriers of the plasma caused by the charge carriers hitting the end walls of the container. The coil windings, being conveniently arranged outside the container of the gas laser, should however enclose the region of the axes of the multipole field emanating from the poles.
Another way of generating an electromagnetic alternating field in the container consists in providing two or more than two electrodes in the container and of making such electrodes part of an electric oscillating circuit which is fed by a frequency generator. In this case, the gas discharge and the resulting generation of plasma take place between the electrodes. In order to permit extraction of laser radiation in this region, the electrodes are conveniently provided with a hole, a recess, perforation or a similar passage in the region of each axis of the multipole field, through which the laser light can pass and eventually leave the container through an extraction window.
A combination of the described two possibilities, which employs a mixture of azimuthal and longitudinal excitation of the plasma (claim 22) is likewise possible and provides the advantage to increase the injected power.
The electromagnetic alternating field may be a LF field, a HF field or a microwave field. The frequency of the electromagnetic alternating field should be at least 50 Hz, although frequencies from the KHz range up to the GHz range are preferred.
The source of the electromagnetic alternating field may also consist of a frequency-adjusted and, thus, resonant, cavity which encloses the spatial region of the laser-active plasma, and which is part of a HF generator or a microwave generator. The basic structures of cavity resonators suited for this purpose are known in microwave technology.
The invention is suited for both, continuously operated lasers (CW lasers) and pulsed lasers. In the case of a pulsed laser it is of advantage to provide means for the permanent generation of gas discharges in combination with means for the pulsed energy supply into the plasma so that the pulsed energy supply into the plasma, which eventually produces the population inversion in the higher energy levels, can be superposed upon the permanent gas discharge, especially a HF frequency or microwave frequency discharge. The permanent HF discharge leads to higher pre-ionisation of the gas and, as a result thereof, to especially homogeneous plasmas. This embodiment of the invention is especially suited for UV lasers.
For pulsed operation of the laser, electrodes are conveniently provided in the laser container, which limit the length of the plasma column in the container and which are part of a circuit in which a re-chargeable capacitor can be discharged via a quick-acting electronic switch and via the electrodes. Suited for use as a quick-acting electronic switch is a thyratron, a thyristor or a pseudo spark switch. A suitable pseudo spark switch has been disclosed in EP 0 324 817 B2. Instead of the electrodes, a coil surrounding the laser container may also be provided through which the capacitor of the circuit is discharged.
Suited as filling gases for the laser container are, above all, rare gases or mixtures of rare gases from the group of helium, neon, argon, xenon and krypton, and in addition carbon dioxide, each with the usual admixtures. A carbon dioxide laser preferably further contains nitrogen and helium or nitrogen and neon, and is used for generating a laser radiation with wave lengths of 9.4 xcexcm and 10.4 xcexcm. For higher powers, the gas fill in the CO2 laser is conveniently circulated and cooled, the gas flowing through that part of the container in which the gas discharge occurs, either in longitudinal or in transverse direction. In the case of a rare-gas fill, especially an argon ion laser, the high electron temperatures (kinetic electron energy) rendered possible by the invention may also excite laser transitions in single-charge or multiple-charge ions, whereby a particularly short-wave laser radiation can be produced. A further increase of frequency is possible by doubling or multiplying the frequency of the emitted laser radiation by means known from non-linear optics. This is so because non-linear optic crystals permit harmonics of the radiation irradiated into the crystal to be produced, a phenomenon that is known as such for doubling the frequency in UV lasers. For further details regarding the technique of frequency multiplication, reference is made to Y. R. Shen: xe2x80x9cThe principles of non-linear opticsxe2x80x9d, Wiley Publishers, N.Y., 1984.
The high electron temperatures rendered possible by the invention, and the frequency multiplication technique make is possible to enter a field in which gas lasers according to the invention work as roentgen lasers. Such lasers are particularly well suited for working and producing very fine structures, including difficult medical operations, especially on eyes, and for treatments on skin.